The present invention relates to a method and apparatus for examining and indicating the status of the cardiovascular system of a subject. The method and apparatus are both particularly useful for measuring the systemic vascular resistance, and also for measuring the cardiac output of the subject, by detecting the subject's arterial pressure and processing same in a particular manner as described below.
A number of measurements relating to a subject's cardiovascular system are useful in assessing its status, these measurements including the mean arterial pressure (MAP), central venous pressure (CVP), cardiac output (CO), and the systemic vascular resistance (SVR), sometimes called the total peripheral resistance (TPR). These measurements have the following relationships: MAP-CVP.apprxeq.CO.times.SVR; this relationship will be recognized as the cardiovascular equivalent of Ohm's law of electricity (E=I.times.R). The mean arterial pressure (MAP) can be obtained by electrical damping or by calculation of the following relationship: EQU MAP=DP+1/3(SP-DP)
wherein DP=diastolic pressure, and SP=systolic pressure. The arterial "pulse pressure" is the difference between the systolic and diastolic pressure, and is dependent on the stroke volume, arterial capacitance, and run-off.
The arterial pressure is commonly measured directly by one of several known techniques, including both non-invasive techniques such as by the use of the Riva-Rocci occlusive cuff, and invasive techniques such as by the use of catheters placed in various arteries, particularly a radial artery or a femoral artery. Cardiac output (CO) is usually measured by a thermal dilution or dye dilution technique; and the systemic vascular resistance (SVR) is usually derived from these measurements according to the above relationship.
A serious drawback in the above techniques for assessing the status of a subject's cardiovascular system is the invasive nature of the procedures used today for measuring cardiac output. The cardiac output, being the amount of blood pumped to the peripheral circulation by the heart per minute, reflects the status of the entire circulatory system, not just the heart, and is an important measurement in assessing the status of the patient's cardiovascular system. The most common technique used today is the thermal dilution procedure employing the Swan-Ganz catheter, in which a cold or iced indicator (e.g. a 5% dextrose solution in water at a temperature of 4.degree. C.) is injected, usually into the heart's right atrium, and the filling pressures of both the right and left sides of the heart are measured by an introduced catheter. Another indicator dilution method uses a green dye and involves the rapid injection of a precise amount of the dye into the central venous circulation, the indicator passing rapidly through the heart and lungs into the arterial circulation where it is detected by sampling arterial blood and passing it through a densitometer. These known techniques are invasive, involve complicated procedures requiring highly-skilled personnel, and are not suited for continuous monitoring.
The above difficulties in the presently known techniques for measuring cardiac output also apply to the measurement of systemic vascular resistance, since that measurement is usually derived (from the above-described relationship) after the mean arterial pressure and the cardiac output have been calculated or otherwise determined.